1. Field of the Invention
The present invention relates to a gas detection apparatus that detects a change in a concentration of a specific gas in an environment by use of a gas sensor element, and to an automatic ventilation system for a vehicle.
2. Prior Art
Conventionally, there has been known a gas sensor element which utilizes Ph-phthalocyanine or a metal oxide semiconductor such as WO3 or SnO2 and whose resistance changes with concentration of a specific gas, such as an oxidative gas (e.g., NOx) or a reducing gas (e.g., CO, HC (hydrocarbon)) present in an environment. Further, there has been known a gas detection apparatus which utilizes such a gas sensor and detects change in concentration of a specific gas on the basis of change in the sensor resistance. Moreover, there have been known various types of control systems using such a gas detection apparatus; e.g., an automatic ventilation system for a vehicle in which a flap is opened and closed in accordance with the degree of contamination of air outside the vehicle compartment in order to effect switching between introduction of external air into the compartment and recirculation of air inside the compartment, and a system which detects contamination of air within a room caused by, for example smoking, and controls an air purifier.
Examples of such gas detection apparatuses utilizing a gas sensor element include a gas detection apparatus which detects a gas by means of differentiating a signal output from a gas sensor element; a gas detection apparatus in which a differentiated analog value is converted to a digital value, which is subjected to digital differentiation so as to obtain a second order derivative for detection of a gas; and a gas detection apparatus which detects a gas by comparing a sensor signal with an integral value obtained through integration of the sensor signal.
However, in gas detection apparatuses which utilize a gas sensor element whose electrical characteristics such as sensor resistance change in accordance with concentration of a specific gas, the electrical characteristics, such as the resistance, of the gas sensor element change in response to not only a change in concentration of the specific gas, but also other changes in the environment, such as changes in temperature, humidity, and wind velocity. Therefore, in a gas detection apparatus which utilizes differentiation so as to detect relative change of an output signal, wherein the output signal changes greatly in response to not only changes in concentration of the specific gas, but also other changes in the environment, such as temperature, humidity, and wind velocity, mere detection of relative change of the output signal fails to clearly distinguish the case in which such a change has occurred in response to a change in concentration of a specific gas from the case in which such a change has occurred in response to a disturbance such as a change in humidity. In the case in which a first order derivative or a second order derivative of the output signal of the gas sensor element is used as described above, a point in time at which gas concentration has changed, such as a point in time at which the gas concentration has suddenly increased, the change can be detected. However, difficulty is encountered in determining the amount by which the gas concentration has changed, or in determining a change in the gas concentration after that point in time and a later point in time at which the gas concentration decreases.
Meanwhile, in a gas detection apparatus which detects a gas by comparing a sensor signal with an integral value of the sensor signal, change in the integral value is delayed with respect to change in concentration of a specific gas. Therefore, in the case in which the gas detection apparatus is designed to increase its sensor output value with the gas concentration, once the concentration of the specific gas starts decreasing, there may arise a reverse state in which the integral value becomes greater than the sensor output value. Therefore, when the concentration of the specific gas increases after the above-described decrease, the gas detection apparatus fails to properly detect change in concentration of the specific gas (e.g., detection timing involves a delay), because the integral value is greater than the sensor output value having started increasing, with the result that the increase in concentration of the specific gas cannot be detected immediately.
Japanese Patent Application Laid-Open (kokai) No. H1-199142 discloses a gas detection apparatus which tracks change in a sensor output over time; stores, as a reference output, a sensor output corresponding to the cleanest atmosphere; after storage, gradually changes the reference output toward a side corresponding to a contaminated atmosphere; and when the changed reference output exceeds the actual sensor output, changes the reference output so as to correspond to the actual sensor output. In the invention, the rate of increase in the reference output is previously set to correspond to change in the sensor output caused by changes in temperature, humidity, etc., whereby gas detection is rendered possible even when temperature or humidity changes.
In the invention described in Japanese Patent Application Laid-Open No. H1-199142, the reference output is changed gradually at a constant rate with respect to time, irrespective of the magnitude of change in the sensor output. However, change in concentration of a specific gas is not constant and cannot be predicted. Take, for example, the case in which the gas concentration changes gradually toward the side corresponding to the contaminated atmosphere. In this case, when the set rate of increase in the reference output is high, the reference output becomes closer to the side corresponding to the contaminated atmosphere as compared with the sensor output, even though the sensor output also changes toward the side corresponding to the contaminated atmosphere, whereby the reference output is changed to correspond to the sensor output. Accordingly, even though the gas concentration has increased gradually to eventually reach a high level, such an increase in gas concentration cannot be detected correctly and quickly, because no difference is produced between the sensor output and the reference output.
Further, since the reference output is gradually changed; e.g., increased linearly, irrespective of the sensor output, the following problem may occur in the case in which a high gas concentration is maintained for a long period of time; e.g., in a long tunnel, so that a high sensor output is output continuously. In such a case, since the reference output gradually increases irrespective of the obtained sensor output, the difference between the sensor output and the reference output decreases, and eventually, the gas concentration is erroneously determined to be low.